I would say that vibration resonance, as distinct from sound resonance, could be fixed by either a flex connection or a different stiffness of mount or perhaps a mass clamped to the pipe somewhere.

A noise resonance can only be fixed by changing the length or diameter of the pipe, basically. In industrial applications there are duct resonators and other tricks to dampen resonance.

Sometimes with a new muffler, the muffler will "break in" as whatever sound damping material inside (if any) settles in, etc.

Rgds,
Chris W.
'99 E300D, 407K

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